The Royals signed Diaz to a minor-league contract Friday that includes an invitation to spring training.
Diaz is expected to report to Royals camp next week. The 35-year-old appeared in 106 regular-season contests for the Padres last season, slashing .204/.270/.337 with nine long balls. Unless the Royals carry three catchers on their Opening Day roster, Diaz is likely ticketed for Triple-A Omaha.